The present invention relates to a process for suppressing the onset of room temperature grain growth and instability in electrodeposited metals used in semiconductor chips and packages. More particularly, the present invention provides a process for enhancing the electromigration resistance of electroplated metal such as copper by enhancing the microstructure of electroplated metal used in chip wiring and packaging applications.
The thermo-mechanical and metallurgical properties, microstructure, and etching characteristics of a plated copper film depend on the electroplating process used to produce the film. More specifically, these qualities depend upon bath additives, nature of agitation, filtration, current density, and the nature of the electrical contacts used to form the film. The microstructure of plated metals such as copper or gold is known to be unstable at room temperature. Room temperature grain recovery and grain growth processes are difficult to control, yet must be suppressed.
Uncontrolled grain recovery and grain growth may cause defects during subsequent processes. For example, the step of etching a plated metal film surface during the grain growth period may result in the etchant species selectively attacking the active grain boundaries. Thus, the grain boundary regions are etched faster than desired. The resulting rough metal surface may present difficulties in subsequent processes, such as forming another film on the metal film surface. A dielectric film may be deposited onto the metal film surface, for example, in a common device application. A rough metal surface may present difficulties in obtaining the desired qualities of the dielectric film being deposited over the etched copper. For example, when the film thickness of the dielectric film is being monitored as deposited using ellipsometric processes, the accuracy of the ellipsometric monitoring is reduced due to anomalous scattering from the highly decorated grains. This reduced accuracy is particularly troublesome when the dimensions of the grains of the plated film are comparable to those of the wavelength of the light used in the optical data collection.
It is also known that the impurity content of deposited metal films depends strongly on the concentration of additives in the plating formulation used in the plating bath. For example, to obtain a specific, desired amount of impurity such as carbon, oxygen, nitrogen, or sulfur, in a plated film, the impurity concentration in the bath formulation must be within a tight range. In order to increase or decrease the amount of impurities produced in the plated film, a different bath formulation normally is required: a bath formulation that contains a correspondingly increased or decreased amount of impurity additives.
What is needed is an improved process for producing an electrodeposited metal film with increased impurity content, such that the impurity content will be nearly independent of the concentration of bath additives. Thus, such a process is one object of the present invention. Another object of the present invention is a process which has suppressed grain growth characteristics. A related object is to provide a process yielding an electrodeposited metal film which has superior metallurgical, electrical, and thermo-mechanical properties.
To achieve these and other objects, and in view of its purposes, the present invention provides a process to suppress the onset of grain growth within an electroplated film before metal etching. Suppressed grain growth or xe2x80x9crecrystallizationxe2x80x9d provides films with superior metallurgical, electrical, and thermo-mechanical properties. The present invention is also directed to a process for producing an electroplated film having an impurity level which is increased relative to that of the prior art, when both the prior art film and the film of the present invention are produced from a bath with an identical impurity concentration.
The present invention is also directed to a process of increasing the amount of overall impurity content in an electroplated film from nominal amounts to much higher levels without modifying or changing the bath formulation used in the electroplating process. The impurity level, more specifically, a laminated impurity layer added to the film, is sufficient to stabilize the microstructure of the plated film and to aid in suppressing grain growth, as described above, but not so high as to create a multi-layer contact resistance problem. The present invention provides a method for laminating impurities into a metal film within a discrete region having a higher impurity concentration than that of a bulk film electrodeposited from the same solution. Therefore, the present invention provides a process for producing an electroplated copper film with an increased resistance to grain growth and which has a higher impurity content than previously obtainable from a bath with a given impurity concentration.
The process of the present invention comprises the steps of: (a) providing a seed layer at least indirectly on a substrate, the seed layer having an exposed surface; (b) immersing the substrate in a plating solution; (c) electrodepositing a copper-containing film on the exposed surface of the seed layer, the copper-containing film having a first surface; (d) maintaining the substrate in an immersed state within the plating solution; (e) electrodepositing a further copper-containing film from the plating solution onto the first surface; (f) removing the substrate from the plating solution; and (g) drying the substrate.